The present invention relates to a process for the epoxidation of olefins, in which the exit gas stream leaving the reactor is further worked up.
It is known from EP A 100 118 that propene may be reacted with hydrogen peroxide to yield propene oxide if titanium silicalite is used as the catalyst. A secondary reaction which always occurs to a slight extent on the titanium silicalite catalyst is the decomposition of hydrogen peroxide to form molecular oxygen. If it is to be possible to operate the epoxidation process safely on an industrial scale, the oxygen formed must be removed from the reaction system. This is most simply achieved by discharging it with a propene exit gas stream. Such a process is known from EP A 659 473. The process does, however, have the disadvantage that considerable quantities of propene and propene oxide are lost together with the oxygen.
The object of the present invention is accordingly to provide a process for the epoxidation of olefins with which higher product yields may be achieved.
This object is achieved by a process for the catalytic epoxidation of olefins in which in one reaction stage the olefin is reacted with aqueous hydrogen peroxide in an organic, water-miscible solvent in the presence of a titanium silicalite catalyst, wherein an exit gas stream is obtained which contains olefin oxide, unreacted olefin and oxygen and this exit gas stream is brought into contact in an absorption unit with the same solvent as used in the reaction stage and a solvent stream loaded with olefin and olefin oxide is drawn off from the absorption unit and an exit gas stream containing oxygen is discharged.
It has now been found that the losses of olefin and olefin oxide which occur on discharge of the exit gas stream containing oxygen during the epoxidation of olefin with hydrogen peroxide and a titanium silicalite catalyst may be reduced in a simple manner by absorbing the majority of the olefin oxide, olefin and optionally the corresponding alkane with the solvent used for the epoxidation, discharging the oxygen and either returning the solvent stream loaded with olefin oxide and olefin to the reaction stage or passing it to a working up stage downstream from the reaction stage.
In a preferred embodiment, an inert gas stream is additionally introduced into the absorption unit, wherein the inert gas leaves the absorption unit together with the oxygen in the exit gas stream. The quantity of inert gas introduced is here preferably selected as a function of the quantity and composition of the exit gas stream leaving the reaction stage such that the exit gas stream leaving the absorption unit is no longer of an ignitable composition. This embodiment has the advantage that, even in the case of variation in product streams in the overall process, it is very simple constantly to maintain the composition of the gas phase in the absorption unit such that an ignitable mixture cannot occur within the absorption unit, nor may it leave said unit as an exit gas stream.
Suitable inert gases are any gases which dissolve only slightly in the solvent used for epoxidation, do not react with hydrogen peroxide and olefin oxide under the epoxidation reaction conditions and do not form explosive mixtures with oxygen. The inert gas preferably used comprises nitrogen or an inert gas obtained by combustion of a methane-air mixture.
Suitable solvents are any solvents which are not oxidized or are only slightly oxidized by hydrogen peroxide under the selected reaction conditions and dissolve in water in a quantity of greater than 10 wt. %. Preferred solvents are those which are unlimitedly miscible with water. Suitable solvents are alcohols, such as for example methanol, ethanol or tert.-butanol; glycols, such as for example ethylene glycol, 1,2-propanediol or 1,3-propanediol; cyclic ethers, such as for example tetrahydrofuran, dioxane or propene oxide; glycol ethers, such as for example ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether or propylene glycol monomethyl ethers and ketones, such as for example acetone or 2-butanone. Methanol is particularly preferably used as the solvent. Absorption is performed at a total pressure in the range from 1 to 25 bar, preferably at the same pressure as the epoxidation reaction, at which the exit gas containing oxygen is obtained. Absorption may be performed at temperatures between the melting point of the solvent and 100xc2x0 C., preferably in the range from 0 to 60xc2x0 C.
In a particularly preferred embodiment of the present invention, the inert gas stream and exit gas stream are passed countercurrently to the solvent. An absorption unit which is suitable for this embodiment is in particular a column with an inert packing or inserts, wherein the exit gas stream loaded with olefin and olefin oxide and the inert gas stream are fed into the bottom of the column, the solvent is supplied to the top of the column, the exit gas stream is discharged at the top of the column and the solvent stream loaded with olefin and olefin oxide is drawn off from the bottom of the column.
The process according to the invention is suitable for the epoxidation of olefins having 2 to 6 carbon atoms. The epoxidation of propene to yield propene oxide is most highly preferred. The process according to the invention is thus illustrated below using the epoxidation of propene by way of example.